1. Field of the Invention
The present invention relates to a transmissive optical encoder.
2. Description of the Related Art
A common type of transmissive optical encoder includes a light emitting element, a light receiving element, and a rotatable disk provided with a number of slits arranged at a predetermined pitch from each other. In the transmissive optical encoder, light emitted from the light emitting element is projected onto the rotatable disk, such that a pattern of light and shadow is generated, resulting from light passing through the slits and light blocked by portions where no slit is formed. The light receiving element detects changes in the pattern of light and shadow as the rotatable disk is rotating. In this way, an angular position and rotational speed of the object, which moves together with the rotatable disk, can be detected.
Some transmissive optical encoders have a configuration in which the light emitting element and the light receiving element are arranged on opposite sides of the rotatable disk and the rotatable disk is situated between the light emitting element and the light receiving element. In this case, due to the light emitting element and the light receiving element as well as their packaging parts, the optical system tends to occupy a larger space.
In order to avoid this disadvantage, a rotatable disk provided with a light emitting element and a light receiving element arranged on the same side in relation to the rotatable disk has been proposed. For example, JP-A-8-184465 discloses an optical encoder which includes a movable plate provided with slits arranged in a circumferential direction at constant gaps therebetween, a light emitting element provided so as to face the movable plate in a first region, a light receiving element provided so as to face the movable element in a second region, and a projection means provided between the first region and the second region on the opposite side of the light emitting element and the light receiving element in relation to the movable element.
JP-A-2008-82958 discloses an optical encoder which includes a rotatable disk formed with a plurality of tracks including slits, light emitting elements oriented to the respective tracks, a light receiving element situated on a rotational axis of the rotatable disk and on the same side as the light emitting element in relation to the rotatable disk, and a rotational optical waveguide disk which is rotated together with the rotatable disk.
JP-A-5-141960 discloses an optical rotational angle detection device which includes an optical fiber serving as an optical waveguide, one end of which is oriented to a light source, and the other end of which is oriented to an optical position sensing element. The optical fiber is supported by a rotatable member coupled to a rotational axis so as to be rotated together with the rotational axis.
However, the optical encoder disclosed in JP-A-8-184465 is configured such that the projection means forms an equal magnification inverting optical system. This complicates the configuration of the optical system, and it is necessary to accurately position the respective elements of the optical system in relation to one another, in order to ensure sufficient detection accuracy. As a result, the manufacturing cost of the encoder is increased. Further, since it is necessary to maintain positional relationships between the respective elements of the optical system, the usage of the encoder is necessarily limited.
The optical encoders disclosed in JP-A-2008-82958 and JP-A-5-141960 are configured such that the optical waveguide for optically coupling the light emitting element to the light receiving element is moved together with the mechanically rotating part. In particular, when the encoder is used in order to detect movement of the electric motor used for industrial machinery, relatively greater mechanical oscillation is generated due to the electric motor operating at high speed. The oscillation may be transmitted to the optical waveguide via the coupling part. As a result, a gap between the optical waveguide and the light receiving element may change, resulting in impairing the detection accuracy of the encoder.
Accordingly, there is a need for a transmissive optical encoder which can be miniaturized without impairing detection accuracy.